1. Field of the Invention
The present invention is broadly concerned with improved starch-based corrugating adhesives, and a premix used in the formulation of such adhesives, which are characterized by the presence of polyvinyl alcohol (PVA) reacted with a ketone-aldehyde resin so as to stabilize the viscosity of the premix, thereby avoiding gelation problems common with PVA-supplemented corrugating adhesives. More particularly, it is concerned with such adhesives and premixes which preferably include fully hydrolyzed PVA reacted with acetone-formaldehyde or urea-formaldehyde resins in the absence of starch and borax to achieve a stabilized PVA in both the premix and final adhesives.
2. Description of the Prior Art
Fabricators of corrugated board or paperboard have long made use of starch-based adhesives. Generally speaking, these adhesives include gelatinized starch, ungelatinized starch, alkali and borax in a water base. Under conditions of heat and pressure during board manufacture, the ungelatinized starch fraction rapidly gelatinizes, thereby absorbing water and rapidly increasing the viscosity and adhesivity of the composition. The presence of borax increases the level of viscosity during the final gelatinization process.
In actual practice, such corrugating adhesives are commonly made using primary and secondary mixers. In the primary mixer, the gelatinized starch fraction is prepared, with the addition of caustic. The secondary mixer normally contains water, borax and ungelatinized starch. The two fractions are the mixed together and may be supplemented with various additives such as cross-linking resins or wax. Such additives may be supplied to the board manufacturer in the form of premixes designed for direct addition to the two starch fractions mixed during final adhesive preparation.
It has also been known in the past to supplement corrugating adhesives with polyvinyl alcohol. PVA in this context is normally added to the adhesive as a finely divided, particulate solid, and is practically insoluble in water at room temperature. At elevated temperatures, however, the PVA becomes soluble and enhances the adhesive properties of the final compositions.
Although the advantages of PVA in corrugated adhesives are known, a number of problems remain. Thus, the use of solid, particulate cold water insoluble PVA does not significantly improve the production rate of corrugating machinery, because of the necessity of elevating the temperature of the PVA so that it becomes soluble.
It has also been suggested to prehydrate the normally cold water insoluble PVA, by mixing the material in warm water. When such hydrated PVA is added to the adhesive, however, problems arise because of premature gelling and reaction with the starch and/or borax constituents of the adhesive.
U.S. Pat. Nos. 4,673,698, 4,600,739, and 4,677,145 describe adhesive and premix systems making use of cold water soluble, partially hydrolyzed PVA having a degree of hydrolysis of less than 91%. According to these patents, such PVA can be employed by carefully controlling the amount of PVA in relation to the boric acid component and taking other steps to insure that the gelation problem is ameliorated.
The water solubility of PVA is directly related to the degree of hydrolysis thereof. PVA is commercially available in a number of hydrolysis grades, e.g., super hydrolyzed (99.3%+ hydrolysis), fully hydrolyzed (98%+ hydrolysis), intermediate hydrolyzed (91-98% hydrolysis), and partially hydrolyzed (85-91% hydrolysis).